Methods and apparatuses for two-qubit gate reduction in quantum circuits

ABSTRACT

The disclosure describes a method, an apparatus, a computer-readable medium, and/or means for reducing two-qubit gates in quantum circuits may include receiving a netlist including information relating to a first plurality of two-qubit quantum gates that form the quantum circuits, performing a controlled gate cancellation operation on the information relating to a first plurality of two-qubit quantum gates to produce a second plurality of two-qubit quantum gates that is functionally equivalent to the first plurality of two-qubit quantum gates, wherein a first number of two-qubit quantum gates in the first plurality of two-qubit quantum gates is larger than a second number of two-qubit quantum gates in the second plurality of two-qubit quantum gates, generating a new netlist containing information about the second plurality of two-qubit quantum gates, and providing the new netlist to implement a functionality of the quantum circuits based on the second plurality of two-qubit quantum gates.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to and the benefit from U.S.Provisional Patent Application No. 62/776,634, entitled “METHODS ANDAPPARATUSES FOR TWO-QUBIT GATE REDUCTION IN QUANTUM CIRCUITS,” and filedDec. 7, 2018, the contents of which are incorporated herein by referencein their entirety.

BACKGROUND OF THE DISCLOSURE

Aspects of the present disclosure generally relate to circuitoptimization, and more specifically, to an automated optimization oflarge-scale quantum circuits with continuous parameters.

Large-scale quantum circuits may be implemented in different ways. Theuse of trapped atomic ions is one of the quantum information processing(QIP) approaches that has delivered universal and fully programmablequantum computing machines. Trapped atomic ions are also a leadingplatform for quantum information networks (QINs). Systems or networksbased on trapped atomic ions that can improve the overall communicationsof such systems or networks are desirable.

It is therefore desirable to develop efficient techniques that allow forbetter optimization (e.g., smaller number of gates) of large-scalequantum circuits in various types of QIP systems, including QIP systemsthat are based on trapped atomic ions as well as those that are basedon, for example, superconducting circuits.

SUMMARY OF THE DISCLOSURE

The following presents a simplified summary of one or more aspects inorder to provide a basic understanding of such aspects. This summary isnot an extensive overview of all contemplated aspects, and is intendedto neither identify key or critical elements of all aspects nordelineate the scope of any or all aspects. Its purpose is to presentsome concepts of one or more aspects in a simplified form as a preludeto the more detailed description that is presented later.

Described herein are techniques for automated optimization oflarge-scale quantum circuits with continuous parameters. For example,aspects of this disclosure describe the implementation of automatedmethods for optimizing quantum circuits of the size and type expected inquantum computations that outperform classical computers. The techniquesdescribed herein illustrate how to handle continuous gate parameters andcan be applied to a collection of fast algorithms capable of optimizinglarge-scale quantum circuits. These techniques can provide betteroptimization in significantly less time than previous approaches, whilemaking minimal structural changes so as to preserve the basic layout ofthe underlying quantum algorithms. The results provided by thesetechniques help bridge the gap between computations that can be run onexisting quantum computing hardware and more advanced computations thatare more challenging to implement in quantum computing hardware but arethe ones that are expected to outperform what can be achieved withclassical computers.

In an example, a method for optimizing quantum circuits is describedthat includes receiving a netlist containing information about a firstlist of quantum gates that form the quantum circuits, performing aphase-polynomial reduction operation on the information about the firstlist of quantum gates to produce a second list of quantum gates that hasfunctional equivalence to the first list of quantum gates, acomputational cost, such as a number of quantum gates, in the secondlist of quantum gates being smaller than a computational cost, such as anumber of quantum gates, in the first list of quantum gates, generatinga new netlist containing information about the second list of quantumgates, and providing the new netlist to implement a functionality of thequantum circuits using the second list of quantum gates.

In some implementations, a method for reducing two-qubit gates inquantum circuits may include receiving a netlist including informationrelating to a first plurality of two-qubit quantum gates that form thequantum circuits, performing a controlled gate cancellation operation onthe information relating to a first plurality of two-qubit quantum gatesto produce a second plurality of two-qubit quantum gates that isfunctionally equivalent to the first plurality of two-qubit quantumgates, wherein a first number of two-qubit quantum gates in the firstplurality of two-qubit quantum gates is larger than a second number oftwo-qubit quantum gates in the second plurality of two-qubit quantumgates, generating a new netlist containing information about the secondplurality of two-qubit quantum gates, and providing the new netlist toimplement a functionality of the quantum circuits based on the secondplurality of two-qubit quantum gates.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended drawings illustrate only some implementation and aretherefore not to be considered limiting of scope.

FIG. 1 is a diagram that illustrates an example of a quantum circuit inaccordance with aspects of this disclosure.

FIG. 2 is a diagram that illustrates an example of a quantum circuit forphase polynomial representation in accordance with aspects of thisdisclosure.

FIG. 3 is a diagram that illustrates examples of rules for Hadamard gatereduction in accordance with aspects of this disclosure.

FIG. 4 is a diagram that illustrates examples of commutation rules inaccordance with aspects of this disclosure.

FIG. 5 is a diagram that illustrates an example of an optimizationsubroutine with rotation merging using phase polynomials in accordancewith aspects of this disclosure.

FIG. 6 is a diagram that illustrates a subcircuit amenable to phasepolynomial representation in accordance with aspects of this disclosure.

FIG. 7 is a diagram that illustrates examples of gate count preservingrewriting rules in accordance with aspects of this disclosure.

FIG. 8 is a diagram that illustrates examples of gate count reducingrewriting rules in accordance with aspects of this disclosure.

FIG. 9 is a diagram that illustrates an example of furthersimplification of the circuit in FIG. 5 in accordance with aspects ofthis disclosure.

FIG. 10 is a diagram that illustrates examples of Toffoli gateimplementations in accordance with aspects of this disclosure.

FIG. 11 is a chart that illustrates examples of total gate counts forthe approximate quantum Fourier transform (QFT), Quipper library adder,and Fourier-based adders (QFA) in accordance with aspects of thisdisclosure.

FIG. 12 is a chart that illustrates examples of numbers of CNOT gatesfor Quipper library adders in accordance with aspects of thisdisclosure.

FIG. 13 is a table that illustrates results for light optimization ofadder circuits in accordance with aspects of this disclosure.

FIG. 14 is a table that illustrates results for heavy optimization ofQuipper library adder in accordance with aspects of this disclosure.

FIG. 15 is a chart that illustrates examples of total gate counts forproduct formula algorithms in accordance with aspects of thisdisclosure.

FIGS. 16A and 16B are tables that illustrate results for optimization ofproduct formula algorithms in accordance with aspects of thisdisclosure.

FIGS. 17A-17C are tables that illustrate comparisons of T-paroptimization with light and heavy optimization for different algorithmsin accordance with aspects of this disclosure.

FIG. 18 is a block diagram that illustrates an example of a device forperforming automated optimization of large-scale quantum circuits withcontinuous parameters in accordance with aspects of this disclosure.

FIG. 19 is a flow chart that illustrates an example of a method forautomated optimization of large-scale quantum circuits with continuousparameters in accordance with aspects of this disclosure.

FIG. 20 is a block diagram that illustrates an example of atrapped-ion-based quantum information processing (QIP) system forimplementing optimized large-scale quantum circuits in accordance withaspects of this disclosure.

FIG. 21 is a diagram that illustrates examples of rules for 2-qubitquantum gate reduction in accordance with aspects of this disclosure.

FIGS. 22A and 22B are diagrams that illustrate examples of rules forcontrolled gate cancellation in accordance with aspects of thisdisclosure.

FIG. 23 shows diagrams of equivalent quantum circuits in accordance withaspects of this disclosure.

FIG. 24 is a flow chart that illustrates an example of a method forreducing two-qubit quantum gates in quantum circuits in accordance withaspects of this disclosure.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of various configurations and isnot intended to represent the only configurations in which the conceptsdescribed herein may be practiced. The detailed description includesspecific details for the purpose of providing a thorough understandingof various concepts. However, it will be apparent to those skilled inthe art that these concepts may be practiced without these specificdetails. In some instances, well known components are shown in blockdiagram form in order to avoid obscuring such concepts.

The disclosure describes the implementation of automated techniques foroptimizing quantum circuits of the size and type expected in quantumcomputations that outperform classical computers. The disclosure showshow to handle continuous gate parameters and report a collection of fastalgorithms capable of optimizing large-scale quantum circuits. For thesuite of benchmarks considered, the techniques described can obtainsubstantial reductions in gate counts. In particular, the techniques inthis disclosure provide better optimization in significantly less timethan previous approaches, while making minimal structural changes so asto preserve the basic layout of the underlying quantum algorithms. Theresults provided by these techniques help bridge the gap betweencomputations that can be run on existing quantum computing hardware andmore advanced computations that are more challenging to implement inquantum computing hardware but are the ones that are expected tooutperform what can be achieved with classical computers.

Introduction

Quantum computers or quantum information processing (QIP) systems havethe potential to dramatically outperform classical computers at solvingcertain problems. Perhaps their best-known application is to the task offactoring integers: whereas the fastest known classical algorithm issuperpolynomial, Shor's algorithm solves this problem in polynomialtime, providing an attack on the widely-used RSA cryptosystem.

Even before the discovery of Shor's algorithm, quantum computers wereproposed for simulating quantum mechanics. By simulating Hamiltoniandynamics, quantum computers can study phenomena in condensed matter andhigh-energy physics, quantum chemistry, and materials science. Usefulinstances of quantum simulation are likely accessible to smaller-scalequantum computers than classically-hard instances of the factoringproblem.

These and other potential applications have helped motivate significantefforts toward building a scalable quantum computer. Two quantumcomputing technologies, superconducting circuits and trapped ions, havematured sufficiently to enable fully programmable universal devices,albeit currently of modest size. Several groups are actively developingthese platforms into larger-scale devices, backed by significantinvestments from both industry and government. Thus, quantumcomputations involving tens or even hundreds of qubits are likely to becarried out in the not-too-distant future.

Experimental quantum information processing remains a difficulttechnical challenge, and the resources available for quantum computationwill likely continue to be expensive and severely limited for some time.To make the most out of the available hardware, it is essential todevelop implementations of quantum algorithms that are as efficient aspossible (e.g., implemented with the fewest number of gates).

Quantum algorithms are typically expressed in terms of quantum circuits,which describe a computation as a sequence of elementary quantum logicgates acting on qubits. There are many ways of implementing a givenalgorithm with an available set of elementary operations, and it isadvantageous to find an implementation that uses the fewest resources.While it is important to develop algorithms that are efficient in anabstract sense and to implement them with an eye toward practicalefficiency, large-scale quantum circuits are likely to have sufficientcomplexity to benefit from techniques that enable automatedoptimization.

In this disclosure, various techniques are described that can beimplemented as software tools (e.g., quantum circuit optimizer) forreducing the size of quantum circuits, aiming to improve theirperformance as much as possible at a scale where manual gate-leveloptimization is no longer practical. Since global optimization ofarbitrary quantum circuits is QMA-hard, the approach described in thisdisclosure is to apply a set of carefully chosen heuristics in anautomated fashion to reduce the gate counts, often resulting insubstantial savings.

The optimization techniques described herein can be applied to severaltypes of quantum circuits. Benchmark circuits include components ofquantum algorithms for factoring and computing discrete logarithms, suchas the quantum Fourier transform, integer adders, and Galois fieldmultipliers. Also considered are circuits for the product formulaapproach to Hamiltonian simulation. In each of these cases, the focus ison circuit sizes that are likely to be useful in applications thatoutperform classical computation. The techniques described herein canhelp practitioners understand which implementation of an algorithm ismost efficient in a given application.

While there is existing work on quantum circuit optimization, there islimited work that focuses on automated optimization techniques targetinglarge-scale circuits such as the ones considered here. Moreover, asdescribed in more detail below, extrapolation of previously-reportedruntimes suggests it is unlikely that existing quantum circuitoptimizers would perform well for such large circuits. Directcomparisons between circuits optimized using the proposed techniques ofthis disclosure and other approaches show that the proposed techniquestypically finds smaller circuits in less time. In addition, the proposedtechniques of this disclosure are used for automated optimization ofquantum circuits with continuous gate parameters.

General Discussion

In this disclosure, the problem of efficiently optimizing large-scalequantum circuits is considered, namely those quantum circuits appearingin quantum computations that are beyond the reach of classicalcomputers. The disclosure describes two optimization approaches referredto as Light and Heavy optimization algorithms (e.g., Light version andHeavy version of the optimizer) that can be implemented and performed assoftware solutions (e.g., offline solutions to large-scale quantumcircuit optimization). These algorithms are based on a chosen sequenceof basic optimizations, yet they achieve substantial reductions in thegate counts, improving over more mathematically sophisticated approachessuch as T-par optimization (described in more detail below). Thesimplicity of our approach is reflected in very fast runtimes,especially using the Light version of the optimizer.

The Heavy version of the optimizer demonstrates that even greatercircuit optimization is possible. To further improve the output, it ispossible to modify the routines for reducing R_(z) count by implementingmore extensive (and thus more computationally demanding) algorithms forcomposing stages of CNOT and R_(z) gates, possibly with some Hadamardgates included. Another consideration is to incorporate template-basedand peep-hole optimizations into the algorithms described herein.Another consideration is to expand the set of subcircuit rewriting rules(described in more detail below) and consider the performance of theapproach on other benchmark circuits. Finally, considering the relativecost of different resources (e.g., different types of gates, ancillaqubits) could lead to optimizers that favorably trade off theseresources.

Methods

Details regarding various optimization algorithms proposed in thisdisclosure and their implementation are described below. Throughout thisdisclosure the term g is used to denote the number of gates appearing ina circuit. The Background section below provides definition of notationsused throughout this disclosure. The section titled Representations ofquantum circuits describes three distinct representations of quantumcircuits that can be employed in connection with the techniquesdescribed herein. In section titled Pre-processing, a pre-processingstep is described that can be used with the various optimizationalgorithms described herein. In section titled Optimization subroutines,several subroutines are described that form the basic building blocks ofthe approach proposed in this disclosure. Moreover, the section titledGeneral-purpose optimization algorithms explains how these subroutinesare combined to form various versions of the optimization algorithms.Finally, in the section titled Special-purpose optimizations, twospecial-purpose optimization techniques are presented that can be usedto handle particular types of circuits.

Background

A quantum circuit is a sequence of quantum gates acting on a collectionof qubits. Quantum circuits are conveniently represented by diagrams inwhich horizontal wires denote time evolution of qubits, with timepropagating from left to right, and boxes (or other symbols joining thewires) represent quantum gates. For example, the diagram 100 in FIG. 1describes a simple three-qubit quantum circuit. The circuit in thediagram 100 includes two (2) single-qubit z-rotation gates (110 a, 110b), R_(z)(θ) and R_(z)(θ′), two (2) single-qubit Hadamard gates (120 a,120 b), H, and four (4) two-qubit controlled-NOT gates (130 a, 130 b,130 c, and 130 d), CNOTs.

A simple set of elementary gates is considered for quantum circuitsconsisting of the two-qubit CNOT gates, together with single-qubit NOTgate, single-qubit Hadamard gates, and single-qubit z-rotation gates asshown in the circuit in the diagram 100. Unitary matrices for thesetypes of gates take expressions of the form:

$\begin{matrix}{{{NOT}\mspace{14mu}\text{:=}\mspace{14mu}\begin{pmatrix}0 & 1 \\1 & 0\end{pmatrix}},{H\mspace{14mu}\text{:=}\mspace{14mu}\frac{1}{\sqrt{2}}\begin{pmatrix}1 & 1 \\1 & {- 1}\end{pmatrix}},{{R_{z}(\theta)}\mspace{14mu}\text{:=}\mspace{14mu}\begin{pmatrix}e^{{- i}\;\theta\text{/}2} & 0 \\0 & e^{i\;\theta\text{/}2}\end{pmatrix}},{{and}\mspace{14mu}{CNOT}\mspace{14mu}\text{:=}\mspace{14mu}\begin{pmatrix}1 & 0 & 0 & 0 \\0 & 1 & 0 & 0 \\0 & 0 & 0 & 1 \\0 & 0 & 1 & 0\end{pmatrix}},} & (1)\end{matrix}$where θ ∈ (0, 2π] is the rotation angle. The phase gate P and the T gatecan be obtained from R_(z)(θ) up to an undetectable global phase asR_(z)(π/2) and R_(z)(π/4), respectively. When the rotation angle isirrelevant, it is possible to denote a generic z-rotation by R_(z).

Although the techniques described herein tend to produce quantumcircuits over the set of H, R_(z), NOT, and CNOT gates, input circuitsthat may include Toffoli gates can also be considered. The Toffoli gate(e.g., gate 1010 a in a diagram 1000 in FIG. 10) is described by themapping |x, y, z

|x, y, z ⊕(x ∧y)

of the computational basis states. The Toffoli gates are also allowed tohave negated controls. For example, the Toffoli gate with its topcontrol negated (e.g., gate 1010 b in the diagram 1000 in FIG. 10) actsas |x,y,z

|x, y, z ⊕(x ∧y)

; and the Toffoli gate with both controls negated (e.g., gate 1010 c inthe diagram 1000 in FIG. 10) acts as |x, y, z

|x, y, z ⊕(x ∧y).

The cost of performing a given quantum circuit depends on the physicalsystem used to implement it. For example, there may be different costconsiderations if the physical system is based on superconductingcircuits or trapped atomic ions. The cost can also vary significantlybetween a physical-level (unprotected) implementation and alogical-level (fault-tolerant) implementation. At the physical level, atwo-qubit gate is typically more expensive to implement than asingle-qubit gate. The techniques described herein accommodate for thisby considering the CNOT gate count and optimizing the number of the CNOTgates in the optimization algorithms.

For logical-level fault-tolerant circuits, the so-called Cliffordoperations (generated by the Hadamard, Phase, and CNOT gates) are oftenrelatively easy to implement, whereas non-Clifford operations can incursignificant overhead. Thus the number of R_(z) gates are also consideredin the optimization algorithms, which try to optimize their count. Infault-tolerant implementations, R_(z) gates are approximated over adiscrete gate set, typically consisting of Clifford and T gates. Optimalalgorithms for producing such approximations are known. The number ofClifford+T gates required to approximate a generic R_(z) gate dependsprimarily on the desired accuracy rather than the specific angle ofrotation, so it is preferable to optimize a circuit before approximatingits R_(z) gates with Clifford+T fault-tolerant circuits.

By minimizing both the CNOT and R_(z) counts, it is possible to performoptimizations targeting both physical- and logical-levelimplementations. Trade-offs between these two goals may be expected, andthere are instances where such trade-offs do occur. However, in thetechniques described herein only optimizations aimed at reducing boththe R_(z) and CNOT counts are considered.

Representations of Quantum Circuits

The following three representations of quantum circuits can be used inconnection with the optimization techniques or optimization algorithmsdescribed herein:

First, a circuit can be stored as a list of gates to be appliedsequentially (a netlist). It is sometimes convenient to specify thecircuit in terms of subroutines (e.g., circuit subroutines), which canbe referred to as blocks. Each block can be iterated any number of timesand applied to any subset of the qubits present in the circuit. Arepresentation using blocks can be especially concise since many quantumcircuits exhibit a significant amount of repetition. A block isspecified as a list of gates and qubit addresses.

The netlist representation can be input and output using a formatproduced by Quipper, a quantum programming language that is used tospecify some of benchmark circuits. This format includes the ability tohandle blocks. Other quantum programming languages and/or formats thathave the same ability may also be used.

Second, a directed acyclic graph (DAG) representation can be used. Thevertices of the DAG are the gates of the circuit and the edges encodetheir input/output relationships. The DAG representation has theadvantage of making adjacency between gates easy to access.

Third, a generalization of the phase polynomial representation of {NOT,CNOT, T} circuits can also be used. Unlike the netlist and DAGrepresentations, this last representation applies only to circuitsconsisting entirely of NOT, CNOT, and R_(z) gates. Such circuits can beconcisely expressed as the composition of a linear Booleantransformation and a diagonal phase transformation. For example, let Cbe a circuit consisting only of CNOT gates and the gates R_(z)(θ₁),R_(z)(θ₂), . . . , R_(z)(θ₁). Then the action of C on the n-qubit basisstate |x₁, x₂ . . . , x_(n)) has an expression of the form|x ₁ x ₂ , . . . ,x _(n)

e ^(ip(x) ¹ ^(,x) ² ^(, . . . ,x) ^(n) ⁾ |h(x ₁ x ₂ , . . . ,x _(n))

  (2)where h: {0,1}^(n)→{0,1}^(n) is a linear reversible function andp(x ₁ ,x ₂ , . . . ,x _(n))=Σ_(i=1) ^(l)(θ_(i) mod 2π)·f _(i)(x ₁ ,x ₂ ,. . . ,x _(n))  (3)is a linear combination of affine Boolean functions f_(i):{0,1}^(n)→{0,1} with the coefficients reduced modulo 2π. The term p(x₁,x₂ . . . , x_(n)) is referred to as the phase polynomial associated withthe circuit C. For example, the circuit shown in a diagram 200 in FIG. 2having four (4) single-qubit z-rotation gates (210 a, 210 b, 210 c, and210 d) and three (3) two-qubit CNOT gates (230 a, 230 b, and 230 c), canbe represented by the mapping expression:|x,y

e ^(ip(x,Y)) |x⊕y,y),  (4)where p(x,y)=θ₁y+θ₂(x⊕y)+θ₃x+θ₄y. In some instances, the phasepolynomial representation may only be considered for {CNOT, T} circuits,so all θ_(i) in the expression (3) are integer multiples of α/4 and thefunctions f_(i) are linear.

It is possible to convert between any two of the above three circuitrepresentations in time linear in the number of gates in the circuit.Given a netlist, it is possible to build the corresponding DAGgate-by-gate. Conversely, it is possible to convert a DAG to a netlistby standard topological sorting. Conversion between the netlist andphase polynomial representations of {NOT, CNOT, R_(z)} circuits ispossible by using, for example, a straightforward generalizationdescribed in connection with, for example, a meet-in-the middlealgorithm used for fast synthesis of depth-optimal quantum circuits.

Pre-Processing

Before running the main optimization procedures (e.g., optimizationalgorithms), the circuit may be preprocessed to make it more amenable tofurther optimization. Since the optimizer can take, in addition to NOT,CNOT, and Toffoli gates, H and R_(z)(θ) gates, then the pre-processingcan be applied to an input circuit consisting of NOT, CNOT, Toffoli, H,and/or R_(z)(θ) gates. Some examples are the Quipper adders and theT-par circuit benchmarks described in more detail below. For instance,the NOT gates can be pushed as far to the right as possible by commutingthem through the controls of Toffoli gates and the targets of Toffoliand CNOT gates. When pushing a NOT gate through a Toffoli gate control,that control is negated (or the negation is removed if it was initiallynegated). If this procedure leads to a pair of adjacent NOT gates, theNOT gates can be removed from the circuit. If no such cancellation isfound, the control negation changes may be reverted and the NOT gate maybe moved back to its original position. Moreover, the commutationrelation between the NOT gate and the H gate is such that NOT followedby H is H followed by a Z gate, where a Z gate is R_(z)(θ=π). Also, NOTfollowed by R_(z)(θ) is R_(z)(−θ) followed by NOT. Therefore, for thestandard set of gates that consists of NOT, CNOT, Toffoli, H, and R_(z)gates, together with the details described below in connection withreducing all affine functions of phases to linear functions by using NOTpropagation through CNOT and R_(z) gates, the NOT gates can be moved tothe beginning or the end of a given circuit as part of thepre-processing step.

This NOT gate propagation leverages two aspects of the proposedoptimizer or optimization algorithms. First, it is possible to acceptToffoli gates that may have negated controls and optimize theirdecomposition into Clifford+T circuits by exploiting freedom in thechoice of T/T^(†) polarities, as described in more detail below. Second,since cancelations of NOT gates simplify the phase polynomialrepresentation (by making some of the functions f_(i) in the phasepolynomial representation (expression 3) linear instead of merelyaffine), such cancelations make it more likely that Subroutines 4 and 5described below will find optimizations (since those subroutines rely onfinding matching terms in the phase polynomial representation).

The complexity of this pre-processing step is O(g) since it is simplymaking a single pass through the circuit.

Optimization Subroutines

The optimization algorithms of this disclosure rely on a variety ofsubroutines that are described below in more detail. For each of them,the worst-case time complexity as a function of the number of gates g inthe circuit is reported (for simplicity, the dependence on the number ofqubits and other parameters is neglected). However, practical-casesoftware runtime can be optimized by carefully ordering and restrictingthe subroutines, as discussed further below.

Subroutine 1—Hadamard Gate Reduction

Hadamard gates do not participate in phase polynomial optimization(Subroutines 4 and 5 below) and also tend to hinder gate commutation.Thus, the circuit identities or rules pictured in a diagram 300 in FIG.3 are used to reduce the Hadamard gate count. The identities or rulesinclude rules 310 a and 310 b at the bottom of the diagram 300 and rules320 a, 320 b, and 320 c at the top of the diagram 300. The rules 310 aand 310 b can be applied even if the middle CNOT gate is replaced by acircuit with arbitrarily many CNOT gates, provided that all of themshare the target of the original CNOT gate.

Each application of these rules reduces the H count by up to 4. For agiven Hadamard gate, the DAG representation can be used to check inconstant time whether it is involved in one of these circuit identities.Thus, it is possible to implement this subroutine with complexity O(g)by making a single pass through all Hadamard gates in the circuit.

Subroutine 2—Single-Qubit Gate Cancelation

Using the DAG representation of a quantum circuit, it is generallystraightforward to determine whether a gate and its inverse areadjacent. If so, both gates can be removed to reduce the gate count.More generally, it is possible to cancel two single-qubit gates U andU^(†) that are separated by a subcircuit A that commutes with U. Ingeneral, deciding whether a gate U commutes with a circuit A may becomputationally demanding. Instead, it is possible to apply a specificset of rules that provide sufficient (but not necessary) conditions forcommutation. This approach is fast and appears to discover manycommutations that can be exploited to simplify quantum circuits.

Specifically, for each gate U in the circuit, the optimizer oroptimization algorithms search for possible cancelations with someinstance of U^(†). To do this, the approach is to repeatedly checkwhether U commutes through a set of consecutive gates, as evidenced byone of the patterns in a diagram 400 in FIG. 4. The diagram 400 includescommutation rules such as rules 410 a, 410 b, and 410 c at the top forcommuting an R_(z) gate to the right, and rules 420 a, 420 b, and 420 cfor commuting a CNOT gate to the right. If at some stage it is notpossible to move U to the right by some allowed commutation pattern,then cancelation of U with a matched U^(†) fails, and the initialconfiguration is restored. Otherwise, cancelation of U with someinstance of U^(†) is successful. The commutation scheme described aboveneed not be applied only with the specific direction to the right butmay also be applied with the specific direction to the left.

For each of the g gates U, the subroutine checks whether it commutesthrough O(g) subsequent positions. Thus the complexity of the overallgate cancelation rule is O(g²). The complexity could be made linear in gby only considering commutations through a constant number of subsequentgates, but this is not found to be necessary in practice.

A slight variation of this subroutine can be used to merge rotationgates, rather than cancel inverses. Specifically, two rotationsR_(z)(θ₁) and R_(z)(θ₂) can be combined into a single rotationR_(z)(θ₁+θ₂) to eliminate one R_(z) gate.

Subroutine 3. Two-Qubit Gate Cancelation

This subroutine is analogous to subroutine 2, except that U is atwo-qubit gate, which is generally CNOT in the circuits underconsideration. Again, the complexity of this subroutine is O(g²), butmay be reduced to O(g) by imposing a maximal size for the subcircuit A.

Subroutine 4. Rotation Merging Using Phase Polynomials

In this case, a subcircuit consisting of the NOT, CNOT, and R_(z) gatesis considered. If two individual terms of its phase polynomialexpression satisfy the expressionf _(i)(x ₁ ,x ₂ . . . x _(n))=f _(j)(x ₁ ,x ₂ , . . . , x _(n))  (5)for some i≠j, then the corresponding rotations R_(z)(θ_(i)) andR_(z)(θ_(j)) can be merged. For example, in the circuit shown in thediagram 200 in FIG. 2, the first and fourth rotations are both appliedto the qubit carrying the value y, as evidenced by its phase polynomialrepresentation. Thus the circuit in the diagram 200 in FIG. 2 goesthrough the transformation shown in a diagram 500 in FIG. 5 in which thetwo rotations are combined. The transformed circuit to the right in thediagram 500 includes R_(z) gates 510 a, 510 b, and 510 c, where R_(z)gate 510 c has the two rotations combined, as well as CNOT gates 530 a,530 b, and 530 c.

In other words, the phase polynomial representation of circuits revealswhen two rotations—in this case, R_(z)(θ₁) and R_(z)(θ₄)—are applied tothe same affine function of the inputs, even if they appear in differentparts of the circuit. Then these rotations can be combined into a singlerotation, improving the circuit. In this particular example, thesimplification could have alternatively been obtained using thecommutation techniques described above; however, that may not generallybe the case. There is a flexibility to place the combined rotation atany point in the circuit where the relevant affine function appears. Forconcreteness, the combined rotation is placed at the first (leftmost)such location.

To apply Subroutine 4, a subcircuit consisting only of {NOT, CNOT,R_(z)} gates needs to be identified. This subcircuit is built one qubitat a time, starting from a designated CNOT gate. For the first qubit ofthis gate, the DAG representation is used to scan through all precedingand subsequent NOT, CNOT, and R_(z) gates that act on this qubit, addingthem to the subcircuit. When a Hadamard gate is encountered at thebeginning or end of the circuit, a termination point is marked andexploring in that direction is stopped (so that each qubit has onebeginning termination point and one ending termination point). For eachCNOT gate between this qubit and some qubit that has not yet beenencountered, an anchor point is marked where the gate acts on thenewly-encountered qubit. This process is carried out with the secondqubit acted on by the initial CNOT gate, and the process is repeatedstarting from every anchor point until no new qubits are encountered.

While the resulting subcircuit consists only of NOT, CNOT, and R_(z)gates, it may not have a polynomial phase representation—specifically,intermediate Hadamard gates on the wires that leave and re-enter thesubcircuit can prevent this. To apply the phase polynomial formalism, itis necessary that this does not happen using the following pruningprocedure. Starting with the designated initial CNOT gate, gates bothbefore and after it in the netlist are successively considered until atermination point is encountered. Note that only CNOT gates need to beconsidered since every NOT and R_(z) gate reached by this process can beincluded, as it does not prevent the phase polynomial expression frombeing applied. If both the control and target qubits of an encounteredCNOT gate are within the termination border, then the procedure cancontinue. If the control qubit is outside the termination border but thetarget qubit is inside, the termination point of the target qubit ismoved so that the CNOT gate being inspected falls outside the border,excluding it and any subsequent gates acting on its target qubit fromthe subcircuit. However, when the control is inside the border and thetarget is outside, an exception is made and the termination point is notmoved (although the CNOT gate is not included in the subcircuit). Thisexception gives a larger {NOT, CNOT, R_(z)} subcircuit that remainsamenable to phase polynomial representation, as illustrated in theexample in a diagram 600 in FIG. 6.

In the example specified by the circuit in the diagram 600, it ispossible to start the search from the first CNOT gate acting on the top(q₁) and middle (q₂) qubits (e.g., CNOT gate 630 a). Traversing q₁ tothe left, an H gate is found (e.g., H gate 620 a), where a terminationpoint is marked. Traversing q₁ to the right, two CNOT gates are found(e.g., CNOT gates 630 b, 630 c), one R_(z) gate is found (e.g., R_(z)gate 610 a), and then an H gate is found (e.g., H gate 620 b), where atermination point is marked. Neither of the encountered CNOT gates joinsq₁ or q₂ to the remaining qubit q₃. The same procedure is repeated on q₂from the original CNOT gate (e.g., CNOT gate 630 a). To the left anR_(z) gate is found (e.g., R_(z) gate 610 b) and then an H gate (e.g., Hgate 620 c), where a termination point is marked. Traversing to theright, a CNOT gate is found acting on q₂ and q₃ (e.g., CNOT gate 630 d).This CNOT reveals additional connectivity, so an anchor point is markedon q₃ on the q₃ (target) side of this CNOT gate. Further to the right onthe q₂ wire, there are three more CNOT gates (e.g., CNOT gates 630 b,630 e, and 630 c), none of which reveals additional connectivity, anR_(z) gate (e.g., R_(z) gate 610 c), and finally an H gate (e.g., H gate620 d), where a termination point is marked. Next q₃ is examined bystarting from the aforementioned anchor point. To the left, an H gate(e.g., H gate 620 e) is found with no further connections to otherqubits, and a termination point is marked. To the right, an H gate(e.g., H gate 620 f) is immediately found and a termination point ismarked.

Having built the subcircuit, next is to go through the netlistrepresentation and to prune it. In this pass, the fourth CNOT gateacting on q₂ and q₃ is encountered (e.g., CNOT gate 630 e), where thecontrol is within the border of the subcircuit but the target is not. Inthis case the exception handling scheme described in the pruningprocedure is applied. This ensures that the last CNOT gate in the {NOT,CNOT, R_(z)} region is included, while excluding the fourth CNOT gate(as indicated by the dashed border in the diagram 600). Thus the lastR_(z) gate appearing in the circuit can be relocated to the verybeginning of the circuit on the q₂ line, to the right of the leftmost H,enabling a phase-polynomial based R_(z) merge, as described in moredetail below.

Once a valid {NOT, CNOT, R_(z)} subcircuit is identified, its phasepolynomial can be generated. For each R_(z) gate, it is possible todetermine the associated affine function its phase is applied to and thelocation in the circuit where it is applied. The list of recorded affinefunctions is then sorted. Finally, it is possible to find and merge allR_(z) gate repetitions, placing the merged R_(z) at the first locationin the subcircuit that computes the desired linear function.

This procedure considers O(g) subcircuits, and the cost of processingeach of these is dominated by sorting, with complexity O(g log g),giving an overall complexity of O(g² log g) for Subroutine 4. However,in practice the subcircuits are typically smaller when there are more ofthem to consider, so the true complexity is lower. In addition, whenidentifying a {NOT, CNOT, R_(z)} subcircuit, the process typicallystarts with a CNOT gate that has not yet been included in any of thepreviously-identified {NOT, CNOT, R_(z)} subcircuits, so the number ofsubcircuits can be much smaller than g in practice. If desired, theoverall complexity can be lowered to O(g) by limiting the maximal sizeof the subcircuit.

As a final step, it is possible to reduce all affine functions of phasesto linear functions. This can be accomplished using NOT propagationthrough CNOT and R_(z) gates as follows:

-   -   NOT(a)CNOT(a; b)        CNOT(a; b)NOT(a)NOT(b);    -   NOT(b)CNOT(a; b)        CNOT(a; b)NOT(b);    -   NOT(a)R_(z)(a)        R_(z) ^(†)(a)NOT(a)

Applying this procedure ensures that each affine function x_(i) ₁ ⊕x_(i)₂ ⊕ . . . ⊕x_(i) _(k) ⊕1transforms into the corresponding linearfunction x_(i) ₁ ⊕x_(i) ₂ ⊕ . . . ⊕x_(i) _(k) , thereby improving thechance to induce further phase collisions.

Subroutine 5—Floating R_(z) Gates

In Subroutine 4, the affine functions associated with R_(z) gates aretracked. More generally, all affine functions that occur in thesubcircuit and their respective locations are recorded, regardless ofthe presence of R_(z) gates. Thus it is possible to identify allpossible locations where an R_(z) gate could be placed, not just thoselocations where R_(z) gates already appear in the circuit. In this“floating” R_(z) gate placement picture, three optimizationsub-subroutines can be employed: two-qubit gate cancelations, gate countpreserving rewriting rules, and gate count reducing rewriting rules.

The first of these sub-subroutines is essentially identical toSubroutine 4, except that R_(z) gates are now floatable and the focus ison a specific identified subcircuit. This approach allows to place theR_(z) gates so as to facilitate cancelations by keeping track of allpossible R_(z) gate locations along the way. In particular, if notplacing an R_(z) gate at a particular location will allow two CNOT gatesto cancel, that location is simply removed from the list of possiblelocations for the R_(z) gate and the CNOT cancelation is performedinstead.

Next rewriting rules are applied that preserve the gate count (see e.g.,a diagram 700 in FIG. 7) in an attempt to find further optimizations.For example, the diagram 700 includes gate count preserving rules 710 aand 710 b. While these replacements do not eliminate gates, they modifythe circuit in ways that can enable optimizations elsewhere. Therewriting rules are provided by an external library file, andsubcircuits are identified to which they can be applied using the DAGrepresentation. The replacements are applied only if they lead to areduction in the two-qubit gate count through one more round of theaforementioned two-qubit cancelation subroutine with floatable R_(z)gates. Note that the rewriting rules may be applicable only with certainfloating R_(z) gates at particular locations in a circuit. Thissubroutine uses floating R_(z) gates to choose those combinations ofR_(z) gate locations that lead to reduction in the gate count.

The last sub-subroutine applies rewriting rules that reduce the gatecount (see e.g., a diagram 800 in FIG. 8 for gate count reducingrewriting rules employed in Subroutine 5). The diagram 800 includes gatecount reducing rewriting rules 810 a, 810 b, 810 c, 810 d, and 810 e.These rules can also be provided via an external library file. Sincethese rules reduce the gate count on their own, the rewriting isgenerally performed whenever a suitable pattern is found. The complexityof this three-step subroutine (i.e., the three optimizationsub-subroutines in Subroutine 5) is upper bounded by O(g³) since thenumber of subcircuits is O(g), and within each subcircuit, the two-qubitcancelation (Subroutine 3) has complexity O(g²). The rewriting rules(e.g., the gate count preserving rewriting rules in FIG. 7, the gatecount reducing rewriting rules in FIG. 8) can be applied with complexityO(g) since, as in Subroutine 1, a single pass through the gates in thecircuit suffices. Again, in practice, the number of subcircuits and thesubcircuit sizes are typically inversely related, which lowers theobserved complexity by about a factor of g. The complexity can belowered to O(g²) by limiting the maximal size of the subcircuit. Indeed,the complexity can be further lowered to O(g log g) by limiting themaximal size of the subcircuit A in the two-qubit gate cancelation (thesorting could still have complexity O(g log g)).

To illustrate an example of how this optimization works, consider thecircuit on the right-hand side in the diagram 500 in FIG. 5. Observethat R_(z)(θ₂) (e.g., gate 510 a) may be executed on the top qubit atthe end of the circuit, allowing the first two CNOTs to cancel, leadingto the optimized circuit illustrated on the right-hand side in a diagram900 in FIG. 9, which is simplified even further by including three (3)R_(z) gates 910 a, 910 b, and 910 c, and a single CNOT gate 930 a.

General-Purpose Optimization Algorithms

The optimization algorithms or techniques described in this disclosuresimply apply the various subroutines described above in a carefullychosen order. There are two versions of the optimizer or optimizationalgorithms that are referred to as a Light version (or simply Light) anda Heavy version (or simply Heavy). In general, the Heavy version appliesmore subroutines, yielding better optimization results at the cost of ahigher runtime. The pre-processing step can be used in both Light andHeavy versions of the optimizer.

The Light version of the optimizer applies the optimization subroutinesin the following sequence or order:

-   -   1, 3, 2, 3, 1, 2, 4, 3, 2.

This sequence is then repeated until no further optimization isachieved. This sequence is at least partially based on the principlethat first exposing {CNOT, R_(z)} gates while reducing Hadamard gates(Subroutine 1) allows for a greater reduction in the cancelationroutines (Subroutines 3, 2, 3), and in particular frees up two-qubitCNOT gates to facilitate single-qubit gate reductions and vice versa.Applying the replacement rule (Subroutine 1) may enable more reductionsafter the first four optimization subroutines. Additional single-qubitgate cancelation and merging is then sought (Subroutine 2). This enablesfaster identification of the {NOT, CNOT, R_(z)} subcircuit regions tolook for further R_(z) count optimizations (Subroutine 4), after whichthe process checks for residual cancelations of the gates (Subroutines3, 2).

The Heavy version of the optimizer applies the following sequence ororder:

-   -   1, 3, 2, 3, 1, 2, 5.

Similarly, this sequence is repeated until no further optimization isachieved. The first six steps in the optimization sequence of the Heavyversion of the optimizer are identical to that of the Light version ofthe optimizer. The difference is that in the Heavy version of theoptimizer the approach used takes advantage of floating R_(z) gates(Subroutine 5), allowing locations for the R_(z) gates that admit betterCNOT gate reductions to be found, including the use of gate countpreserving rewriting rules to expose further gate cancelations and gatecount reducing rewriting rules to remove any remaining inefficiency.

The orders or sequences described above are provided by way ofillustration and other orders or sequences can also be used in whichmore or fewer of the subroutines described above are involved.

It is to be noted that the computational overhead incurred due to thecircuit representation conversion tends to be minimal. All conversionscan be done in time linear in the circuit size (see e.g., section titledRepresentations of quantum circuits for detail). Representations arekept consistent only as necessary. In Subroutine 1-Subroutine 3,individual gates can be accessed using the DAG representation to quicklyfind reductions. This allows the update of only the DAG representationto record gate count reductions before continuing with the optimizationprocess. In Subroutine 4 and Subroutine 5, both representations can beupdated concurrently on the fly whenever a reduction is found, keepingboth the DAG and netlist representations consistent. This is usefulsince both routines identify subcircuits that are amenable to reductionsusing the phase polynomial representation. The identification processrequires an up-to-date DAG representation and creating the phasepolynomial representation requires an up-to-date netlist representation.Note that the phase polynomial representation is employed only to aidoptimization in the identified subcircuit; it is not necessary toconvert the phase polynomial representation back to either the netlistor the DAG representation. The phase polynomial representation may thusbe safely purged when the corresponding subcircuit optimization processis finished.

Special-Purpose Optimizations

In addition to the general-purpose optimizer described above (e.g.,general-purpose optimization algorithms), two specialized optimizationscan be employed to improve circuits with particular structures.

optimizer: Some quantum algorithms—such as product formula simulationalgorithms—involve repeating a fixed block multiple times. To optimizesuch a circuit, it may be best to first run the optimizer on a singleblock to obtain its optimized version,

. To find simplifications across multiple blocks, the circuit

² can be optimized and the result can be called

, where

the maximal prefix of

in the optimization of

². The next step is to optimize

³. Provided optimizations only occur near the boundaries between blocks,it is possible to remove the prefix

and the suffix

from the optimized version of

³, and call the remaining circuit

. Assuming that such

,

, and

can be found (which is typically the case in practice), then it ispossible to simplify

^(t) to

^(t-2)

Toffoli decomposition: Many quantum algorithms are naturally describedusing Toffoli gates. The optimizer or optimization algorithms describedin this disclosure can handle Toffoli gates with both positive andnegative controls. Since the goal is to express circuits with respect tothe gate set {NOT, CNOT, H, R_(z)}, the Toffoli gate is decomposed interms of these elementary gates. The techniques described herein takeadvantage of different ways of doing this to improve the quality ofoptimization.

Specifically, the Toffoli gates can be expanded in terms of one- andtwo-qubit gates using the identities shown in the diagram 1000 in FIG.10, keeping in mind that it is also possible to obtain the desiredToffoli gate by exchanging T and Tt in those circuit decompositions(because the Toffoli gate is self-inverse). As described above, theidentities in the diagram 1000 include the Toffoli gate 1010 a, theToffoli gate with top control negated 1010 b, and the Toffoli gate withboth controls negated 1010 c. Initially, the optimizer leaves thepolarity of T/T^(†) gates (i.e., the choice of which gates include thedagger and which do not) in each Toffoli decomposition undetermined. Theoptimizer symbolically processes the indeterminate T and T^(†) gates bysimply moving their locations in a given quantum circuit, keeping trackof their relative polarities. The optimization is considered completewhen movements of the indeterminate T and T^(†) gates cannot furtherreduce the gate count. Finally, it is possible to choose the polaritiesof each Toffoli gate (subject to the fixed relationships between them)with the goal of minimizing the T count in the optimized circuit. Thisminimization is performed in a greedy way, choosing polarities for eachToffoli gate in the order of the appearance of the associated T/T^(†)gates in the nearly-optimized circuit, so as to reduce the T count asmuch as possible.

Overall, this polarity selection process takes time O(g). Afterselecting polarities, it is possible to run Subroutine 3 and Subroutine2, since particular choices of polarities may lead to furthercancelations of the CNOT gates and single-qubit gates that wereotherwise not possible due to the presence of the indeterminate gatesblocking the desired commutations.

Optimization Results

An example implementation of aspects of the optimizer or optimizationalgorithms described herein has been made using the Fortran programminglanguage and tested it using three sets of benchmark circuits. Allresults shown in this disclosure were obtained using a machine with a2.9 GHz Intel Core i5 processor and 8 GB of 1867 MHz DDR3 memory,running OS X El Capitan. It is to be understood that the results areprovided by way of illustration and to show relative performance, andthat the results may vary when performed using different hardwareconfigurations.

As part of the testing various quantum circuits that include componentsof Shor's integer factoring algorithm were considered, namely thequantum Fourier transform (QFT) and the integer adders. Also consideredwere quantum circuits for the product formula approach to Hamiltoniansimulation (PF). In both cases, the focus of the testing was on quantumcircuit with sizes likely to be useful in applications that outperformclassical computation, and ran experiments with different types ofadders and product formulas. Finally, a set of benchmark quantumcircuits were also considered, consisting of various arithmetic circuits(including a family of Galois field multipliers) and implementations ofmultiple control Toffoli gates. Data files describing these circuitsbefore and after optimization are available for comparison.

Correctness of the optimizer or optimization algorithms was checked byverifying the functional equivalence (i.e., equality of thecorresponding unitary matrices) of various test circuits before andafter optimization. Such a test is generally feasible for circuits witha small number of qubits. These tests were performed for all 8-qubitbenchmarks in Table 1 shown in a diagram 1300 in FIG. 13 and Table 2shown in a diagram 1400 in FIG. 14. Table 1 includes light optimizationof adder circuits with the results for an in-place adder based on theQFT (or QFA) on top and for a Quipper library adder at the bottom. Thesetests were also performed for all 10-qubit benchmarks in Tables 3A and3B shown in diagrams 1600 a and 1600 b in FIGS. 16A and 16B,respectively. Table 3A shows optimization of product formula algorithmswith CNOT gate reduction and Table 3B shows optimization of productformula algorithms with R_(z) gate reduction. Software runtimes rangefrom 0.004 s (1st order, n=10) to 0.137 s (6th order, n=100). TheClifford gate reduction ranges from 62.5% for Hadamard and 75% for Phasegates (for the 1st order formula, independent of n) to 75% for Hadamardand 85% for Phase gates (for the 6th order formula, again independent ofn). The notation “(×1000)” indicates that the gate counts for the 1storder formula are in units of thousands (no rounding errors). Thenotation “(L)” denotes the standard Light version of the optimization asdescribed above.

In addition, Tables 4A-4C shown in diagrams 1700 a-1700 c in FIGS.17A-17C represent the results of the optimizer in comparison withfollowing benchmarks: Mod5₄, VBE-Adder₃, CSLA-MUX₃, RC-Adder₆,Mod-Red₂₁, Mod-Mult₅₅, Toff-Barenco_(3 . . . 5), Toff-NC_(3 . . . 5),GF(2⁴)-Mult, and GF(2⁵)-Mult. For example, Tables 4A-4C include T-parcomparisons (described in more detail below), where the names of thebenchmark algorithms are as generally known except that Toff-Barenco andToff-NC are used to denote implementations of multiply-controlledToffoli gates. The notation “(L)” denotes the standard Light version ofthe optimization, while the notation “(H)” denotes the standard Heavyversion of the optimization. The symbol ┤ ├ indicates that there was noimprovement in the Heavy version of the optimization over the Lightversion of the optimization.

QFT and Adders

The QFT is a fundamental subroutine in quantum computation, appearing inmany quantum algorithms with exponential speedup. The standard circuitfor the exact n-qubit QFT uses R_(z) gates, some with angles that areexponentially small in n. It is well understood that one can perform ahighly-accurate approximate QFT by omitting gates with very smallrotation angles. For the purposes of the optimizer or optimizationalgorithms of this disclosure, rotations by angles at most π/2¹³ may beomitted, which ensures sufficient accuracy of the approximate QFT forquantum circuits of the sizes under consideration. These small rotationsare removed before optimization so that their omission does notcontribute to the improvements reported in the results.

In the chart shown in a diagram 1100 in FIG. 11, shows the total gatecount for the approximate quantum Fourier transform (QFT, inset),Quipper library adder, and Fourier-based adders (QFA). The symbols inwhite/black represent gate counts before/after optimization and thesquare/circle/triangle symbols represent gate counts for the Quipperlibrary adder/QFA/QFT, respectively. A savings ratio of larger than 36%is observed for the QFT with 512 or more qubits. The optimization comesentirely from the reduction in the number of R_(z) gates, the mostexpensive resource in a fault-tolerant implementation.

As shown in the chart in the diagram 1100 in FIG. 11, two types ofinteger adders are considered: an in-place modulo 2^(q) adder asimplemented in the Quipper library and an in-place adder based on theQFT (or QFA). The QFA circuits use an approximate QFT in which therotations by angles less than π/2¹³ are removed, as described above.Adders are a basic component of Shor's quantum algorithm for integerfactoring. The results shown report gate counts before and afteroptimization for the Quipper adders and the QFAs for circuits acting on2^(L) qubits, with L ranging from 4 to 11. Adders with L=10, forexample, are used in Shor's integer factoring algorithm for factoring1,024-bit numbers. The related RSA-1024 challenge remains unsolved.

The results of Light (version) optimization of the adder circuits areshown in FIGS. 11 and 13. For the Quipper library adders, the full Lightoptimizer is used. For the QFA optimization, a modified Light optimizeris used instead with the sequence of subroutines 1, 3, 2, 3, 1, 2,omitting the final three subroutines 4, 3, 2 of the standard Lightoptimizer. This was done as no additional gate savings was found fromthose subroutines in small instances (n≤256).

The simplified Quipper library adder outperforms the QFA by a widemargin, suggesting that it may be preferred in practice. For the Quipperlibrary adder, a reduction in the T gate count by a factor of up to 5.2is observed. This reduction is obtained entirely by automated means(i.e., by the optimization algorithms), without using any priorknowledge of the circuit structure. Since Shor's integer factoringalgorithm is dominated by the cost of modular exponentiation, which inturn relies primarily on integer addition, this optimization reduces thecost of executing the overall factoring algorithm by a factor of morethan 5.

The Heavy optimizer was also applied to the QFT and adder circuits. Forthe QFT and QFA circuits, the Heavy setting does not improve the gatecounts. The results of the Heavy optimization for the Quipper adder areshown in Table 2 in the diagram 1400 in FIG. 14. A reduction in the CNOTcount by a factor of 2.7 is found compared to a factor of only 1.7 forthe Light optimization. A chart in a diagram 1200 in FIG. 12 illustratesthe total CNOT counts of the Quipper library adder before optimization(e.g., pre-optimization), after Light optimization, and after Heavyoptimization, showing the reduction in the CNOT count by the two typesof optimization. The square symbols in white/dot patterned/blackrepresent the gate counts in pre-optimization/post-Lightoptimization/post-Heavy optimization, respectively.

Quantum Simulation

A first explicit polynomial-time quantum algorithm for simulatingHamiltonian dynamics has been introduced and the approach was latergeneralized to higher-order product formulas (PFs), resulting inimproved asymptotic complexities. This disclosure reports gate countsbefore and after optimization for the PF algorithms of orders 1, 2, 4,and 6 (for orders higher than 1, the order of the standard Suzukiproduct-formula construction is even). For concreteness, thesealgorithms are implemented for a one-dimensional Heisenberg model in amagnetic field with periodic boundary conditions, evolving the systemfor the time proportional to its size, and the algorithm parameters arechosen to ensure the Hamiltonian simulation error is at most 10⁻³ usingknown bounds on the error of the product formula approximation.

The results of Light optimization of product formula algorithms arereported in Tables 3A and 3B (e.g., FIGS. 16A and 16B) and illustratedin a chart in a diagram 1500 in FIG. 15, where the symbols inwhite/black represent gate counts before/after optimization and thesquare/circle symbols represent gate counts for the second-/fourth-orderformula, respectively. For these product formula algorithms, the Heavyoptimization does not offer a further improvement over the Lightoptimization. The 2nd-, 4th-, and 6th-order algorithms admit a ˜33.3%reduction in the CNOT count and a ˜28.5% reduction in the R_(z) count,roughly corresponding to the reductions relevant to physical-level andlogical-level implementations. The 1st-order formula algorithm did notexhibit CNOT or R_(z) gate optimization. In all product formulaalgorithms, the number of Phase and Hadamard gates reducedsignificantly, by a factor of roughly 3 to 6.

Comparison with Other Approaches

Quantum circuit optimization is a known field. However, prior work oncircuit optimization has not considered large-scale quantum circuits ofthe kind that could outperform classical computers as considered in thisdisclosure. For instance, in the work by Amy, Maslov, and Mosca(“Polynomial-time T-depth optimization of Clifford+T circuits viamatroid partitioning,” IEEE Trans. Comput. AidedDes. Integr. CircuitsSyst., vol. 33, pages 1476-1489 (2014)), the complexity of optimizing ag-gate circuit is O(g³), making optimization of large-scale circuitsunrealistic. There are examples of running times ranging from 0.07 to1.883 seconds for numbers of qubits from n=10 to 35 and gate counts from60 to 368, whereas the optimization algorithms described herein ran fora comparable time when optimizing the Quipper adders up to n=256 witharound 23,000 gates, as shown in Table 1 (FIG. 13). There also are otherexamples of quantum circuit optimization based on peep-hole optimizationusing optimal gate libraries, which are expensive by taking in somecases more than 100 seconds for a 20-qubit, 1,000-gate circuit.

To compare the results produced by using the optimization algorithmsproposed in this disclosure to those reported previously, a weightedcombination of T and CNOT counts is considered. While the T gate can beconsiderably more expensive to implement fault-tolerantly using statedistillation, neglecting the cost of the CNOT gates may lead to asignificant underestimate. Some analysis suggest that a fault-tolerant Tgate may be 46 to 350 times more expensive to implement than a localfault-tolerant CNOT gate, with at least one recommendation regarding thecost ratio being 1:50. The true overhead depends on many details,including the fault tolerance scheme, the error model, the size of thecomputation, architectural restrictions, the extent to which theimplementation of the T gate can be optimized, and whether T stateproduction happens offline so its cost can be (partially) discounted.For a rough comparison, one approach is to work with an aggregate costmetric defined as follows: #T+0.01·log n·#CNOT, where #T is the numberof T gates used, 0.01 accounts for the relative efficiency of the CNOTgate relative to the T gate, n is the number of qubits in thecomputation, and #CNOT is the number of CNOT gates used. Here the factorof log n underestimates the typical cost of performing gates betweenqubits in a realistic architecture (whereas the true cost may be closerto ∛√{square root over (n)} in three dimensions or √{square root over(n)} in two dimensions). Since the approaches described herein preservethe structure of the original circuit, this metric should give aconservative comparison with other approaches (such as the T-parapproach mentioned below) that may introduce long-range gates.Therefore, showing advantage with respect to this aggregate cost canvery crudely demonstrate the benefits of the proposed optimizationtechniques described in this disclosure.

Results from using the techniques described in this disclosure can becompared directly with the results reported by Amy, Maslov, and Mosca,which aim to reduce the T count and T depth using techniques based onmatroid partitioning. That approach is what is referred to in thisdisclosure as T-par. Results from using the techniques described in thisdisclosure are used to optimize a set of benchmark circuits appearing inthat work and the results are compared with the T-par optimization, asshown in Tables 4A-4C (FIGS. 17A-17C).

The benchmark circuits fall into three categories. The first set (Table4A in FIG. 17A) consists of a selection of arithmetic operations. Forthese circuits, better or matching T counts are obtained compared to thebenchmarks while also obtaining much better CNOT counts. Note that thecircuit CSLA-MUX₃ is excluded from the comparison since it is believedT-par optimized it incorrectly. To illustrate the advantage of theoptimization techniques described herein, using the aggregate costmetric described above the proposed techniques reduced the cost of theRC-Adder₆ circuit from 71.91 to 49.70. The improvement in cost is thusby about 31%, mostly due to a reduced T gate count.

The second set of benchmarks consists of multiply-controlled Toffoligates (Table 4B in FIG. 17B). While the proposed optimizer matched the Tcount obtained by the T-par optimizer and substantially reduced the CNOTcount, neither the proposed optimizer nor the previous approaches couldfind the best known implementations. This is not surprising, given thevery different circuit structure employed in the best knownimplementations.

The third set of benchmarks contains Galois field multiplier circuits(Table 4C in FIG. 17C). In an example, the Heavy optimizer is terminatedwhen its runtime exceeds that of the light optimizer by a factor of 200.Such a timeout occurred when applying the Heavy optimizer to the fourlargest instances of the Galois field multiplier circuits. There was noadvantage from the Heavy optimizer over the Light optimizer in the casestested, so the Heavy optimizer was not applied to the four largestinstances (the corresponding entries are left blank in Table 4C). The Tcount again matches that of the T-par optimizer, but the CNOT count ismuch lower, resulting in the circuits that are clearly preferred. Forexample, the optimized GF(2⁶⁴) multiplier circuit has been shown to use180,892 CNOT gates, whereas the optimized implementation resulting fromthe techniques of this disclosure uses only 24,765 CNOT gates; theaggregate cost is thus reduced from 30,168.59 to 18,326.42 despite nochange in the T count, that is, by about 39%. The reduction comes mostlyor solely from the CNOT gates. This comparison demonstrates that thediscrepancy between T count and realistic aggregate cost estimatepredicted in theory is manifested in practice. The efficiency of theLight optimizer allowed the optimization of the GF(2¹³¹) and GF(2¹⁶³)multiplier quantum circuits, corresponding to instances of the ellipticcurve discrete logarithm problem that remain unsolved. Given thereported T-par optimization runtimes, an instance of this size appearsto be intractable for the T-par optimizer.

A new tool for T count optimization have been proposed (e.g., by Heyfronand Campbell). The approach used in this new tool relies on measurementand classical feedback, in contrast to the fully unitary circuitsconsidered in this disclosure. Moreover, this new tool does not provideCNOT counts, making it impossible to give a direct comparison thataccounts for both T and CNOT gates. Again, this new tool solely targetsT count optimization, whereas the techniques of this disclosure departedfrom this simple costing metric. For example, an optimized QFT₄ circuitbased on this new T count optimization tool implements a 4-qubit QFTtransformation using 44 qubits, suggesting that the CNOT gate overheadmust be large. A further significant difference is scalability: whilethe techniques described herein are suitable for and applied to optimizelarge circuits, the new T count optimization tool only treats very smallcircuits—for instance, the largest GF multiplier optimized by that toolis the 7-bit case, whereas the techniques described herein can tackle GFmultipliers with 131 and 163 bits, corresponding to unsolved Certicomchallenges. Another difference is that the techniques proposed hereinuse only those qubit-to-qubit interactions already available in theinput circuits. This enables executing optimized circuits in the samearchitecture as the input circuit, which may be useful for quantumcomputers over restricted architectures. In contrast, the new T countoptimization tool introduces new interactions. Finally, the techniquesproposed herein can handle circuits with arbitrary R_(z) gates, whereasthe new T count optimization tool is limited to Clifford+T circuits.

Overall Performance

Examples of numerical optimization results produced by the optimizationalgorithms described in this disclosure are presented in Table 1, Table2, Tables 3A and 3B, and Tables 4A-4C. These tables contain benchmarksrelevant to practical quantum computations that are beyond the reach ofclassical computers. In Table 1 and Table 2 these are the 1,024- and2,048-qubit QFT and integer adders used in classically-intractableinstances of Shor's factoring algorithm. In Tables 3A and 3B theseinclude all instances with n ≳50, for which direct classical simulationof quantum dynamics is currently infeasible. In Tables 4A-4C these areGalois field multipliers over binary fields of sizes 131 and 163, whichare relevant to quantum attacks on unsolved Certicom ECC Challengeproblems. This illustrates that the proposed optimizer or optimizationalgorithms are capable of handling quantum circuits that aresufficiently large to be practically relevant.

The proposed optimizer or optimization algorithms can be applied moregenerally than previous work on circuit optimization. It readily acceptscomposite gates, such as Toffoli gates (which may have negatedcontrols). It also handles gates with continuous parameters, a usefulfeature for algorithms that naturally use R_(z) gates, includingHamiltonian simulation and factoring. Many quantum informationprocessing technologies natively support such gates, including bothtrapped ions and superconducting circuits, so that the proposed approachdescribed herein may be useful for optimizing physical-level circuits.

Fault-tolerant quantum computations generally rely on a discrete gateset, such as Clifford+T, and optimal Clifford+T implementations of R_(z)gates are already known. Nevertheless, the ability to optimize circuitswith continuous parameters is also valuable in the fault-tolerantsetting. This is because optimizing with respect to a naturalcontinuously-parametrized gate set before compiling into a discretefault-tolerant set will likely result in smaller final circuits.

Finally, unlike previous approaches, the proposed optimizer of thisdisclosure is configured to preserve the structure of the originalquantum circuit. In particular, the set of two-qubit interactions thatare used by the optimized quantum circuit is a subset of those used inthe original quantum circuit. This holds because neither thepre-processing step nor the optimization algorithms introduce any newtwo-qubit gates. By keeping the types of interactions used under control(in stark contrast to, for example, T-par, which dramatically increasesthe set of interactions used), the optimized implementations resultingfrom the proposed optimization algorithms are better suited forarchitectures with limited connectivity. In particular, given a layoutof the original quantum circuit on hardware with limited connectivity,this property allows use the same layout for the optimized quantumcircuit. Moreover, unlike other optimization tools (e.g., the new Tcount optimization tool described above), the proposed optimizer of thisdisclosure does not increase the number of the CNOT gates used. This canbe of relevant practical consideration since a long-range CNOT gate canbe even more expensive than a T gate, and focusing on T optimizationalone may result in circuits whose cost is dominated by CNOT gates.

Referring now to a diagram 1800 in FIG. 18, illustrated is an examplecomputer device 1810 in accordance with an implementation, where thecomputer device 1810 may be used to perform the various optimizationalgorithms described above, including the various optimizationsubroutines and the Light and Heavy versions of the optimizer. Thecomputer device 1810 can represent a single computing device, multiplecomputing devices, or a distributed computing system, for example. Thecomputer device 1810 may be configured to execute or perform theautomated optimization of quantum circuits, including automatedoptimization of large-scale quantum circuits with continuous parameters,as described herein. Moreover, the computer device 1810 may beconfigured to receive information about the quantum circuits (e.g., anetlist) and generate new information (e.g., a new netlist) after theoptimization such that the number of quantum gates needed to implementthe quantum circuits (or the functionality of the quantum circuits) isless than the number of quantum gates originally needed (e.g., prior tothe optimization) to implement the quantum circuits (or thefunctionality of the quantum circuits).

In one example, the computer device 1810 may include a processor 1848for carrying out processing functions associated with one or moreoptimization functions or operations described herein. The processor1848 can include a single or multiple set of processors or multi-coreprocessors. Moreover, the processor 1848 can be implemented as anintegrated processing system and/or a distributed processing system. Inan implementation, for example, the processor 1848 may include a centralprocessing unit (CPU), a graphics processing unit (GPU), a tensorprocessing unit (TPU), or a combination of one or more of these types ofunits, which can be configured to perform one or more of theoptimization functions or operations described herein.

In an example, the computer device 1810 may include a memory 1850 forstoring instructions executable by the processor 1848 for carrying outthe functions described herein. In an implementation, for example, thememory 1850 may correspond to a computer-readable storage medium thatstores code or instructions to perform one or more of the optimizationfunctions or operations described herein.

Further, the computer device 1810 may include a communications component1852 that provides for establishing and maintaining communications withone or more parties utilizing hardware, software, and services asdescribed herein. The communications component 1852 may carrycommunications between components on the computer device 1810, as wellas between computer device 1810 and external devices, such as deviceslocated across a communications network and/or devices serially orlocally connected to computer device 1810. In one example, thecommunications component 1852 may provide communication of informationbetween the computer device 1810 and a quantum information processing(QIP) system such that optimized netlists or the like are provided tothe QIP system once produced by the computer device 1810. In anotherexample, the communications component 1852 may include one or morebuses, and may further include transmit chain components and receivechain components associated with a transmitter and receiver,respectively, operable for interfacing with external devices.

Additionally, the computer device 1810 may include a data store 1854,which can be any suitable combination of hardware and/or software, thatprovides for mass storage of information, databases, and programsemployed in connection with implementations described herein. Forexample, the data store 1854 may be a data repository for operatingsystem 1840 and/or optimization application 1830. In one implementation,the data store 1854 may include memory 1850.

The data store 1854 and/or the memory 1850 may be used to storeinformation associated with pre-optimized circuits, post-optimizedcircuits, intermediate information produced during optimization, theoptimization algorithms, including the subroutines and various versionsof the optimizer, and any of the rules associated with the optimizationoperations described herein.

The computer device 1810 may also include a user interface component1856 operable to receive inputs from a user of the computer device 1810and further operable to generate outputs for presentation to the user.The user interface component 1856 may include one or more input devices,including but not limited to a keyboard, a number pad, a mouse, atouch-sensitive display, a digitizer, a navigation key, a function key,a microphone, a voice recognition component, any other mechanism capableof receiving an input from a user, or any combination thereof. Further,the user interface component 1856 may include one or more outputdevices, including but not limited to a display, a speaker, a hapticfeedback mechanism, a printer, any other mechanism capable of presentingan output to a user, or any combination thereof.

In an implementation, the user interface component 1856 may transmitand/or receive messages corresponding to the operation of the operatingsystem 1840 and/or the optimization application 1830. In addition, theprocessor 1840 may execute the operating system 1840 and/or theoptimization application 1830, and the memory 1850 or the data store1854 may store them. Moreover, the optimization application 1830 may beexecuted based on stored code or instructions to perform one or more ofthe optimization functions or operations described herein. For example,the optimization application 1830 may select between a Light version ofthe optimizer or a Heavy version of the optimizer and may implement theappropriate sequence of subroutines (and any operations within each ofthe subroutines) accordingly.

FIG. 19 is a flow chart that illustrates a process or method 1900 toperform an automated optimization of quantum circuits in accordance withaspects of this disclosure. Aspects of the method 1900 may be performedby hardware and/or software in the computer device 1810 shown in FIG.18.

At block 1905, the method 1900 includes receiving a netlist containinginformation about a first list of quantum gates that form the quantumcircuits. A list of quantum gates may permit a repetition of elementssince quantum circuits may have duplicates of the same quantum gates.

At block 1910, the method 1900 includes performing a phase-polynomialreduction operation on the information about the first list of quantumgates to produce a second list of quantum gates that has functionalequivalence to the first list of quantum gates, a computational cost,such as a number of quantum gates, in the second list of quantum gatesbeing smaller than a computational cost, such as a number of quantumgates, in the first list of quantum gates.

At block 1915, the method 1900 includes generating a new netlistcontaining information about the second list of quantum gates.

At block 1920, the method 1900 includes providing the new netlist toimplement a functionality of the quantum circuits using the second listof quantum gates.

In another aspect of the method 1900, a pre-processing operation may beperformed prior to performing the phase-polynomial reduction operation.The pre-processing operation may be applied to NOT gates, CNOT gates,Toffoli gates, Hadamard gates, and R_(z) gates.

In another aspect of the method 1900, a Hadamard gate reductionoperation may be performed prior to performing the phase-polynomialreduction operation.

In another aspect of the method 1900, a single qubit gate cancelationoperation may be performed prior to performing the phase-polynomialreduction operation.

In another aspect of the method 1900, a two-qubit gate cancelationoperation may be performed prior to performing the phase-polynomialreduction operation.

In another aspect of the method 1900, performing the phase-polynomialreduction operation may include implementing a set of rewriting rules.The set of rewriting rules may include one or both of gate countpreserving rewriting rules or gate count reducing rewriting rules.

In another aspect of the method 1900, one or more gate cancelationoperations or gate reduction operations along with the phase-polynomialreduction operation may be performed iteratively.

In another aspect of the method 1900, a fixed sequence of optimizationoperations that includes the phase-polynomial reduction operation may beperformed iteratively, where the phase-polynomial reduction operation isnot the first optimization operation in the fixed sequence and isperformed only once in the fixed sequence.

FIG. 20 shows a diagram 2000 illustrating an example of a QIP system2005 in accordance with aspects of this disclosure. The QIP system 2005may also be referred to as a quantum computing system, a quantumcomputing network, a computer device, or the like. In an aspect, the QIPsystem 2005 may be used to implement or perform quantum computingoperations or algorithms for which the fidelity of the quantum gatesbeing implemented depends on the ability to have a tuned, stable laseroutput applied to, for example, trapped ions used as quantum bits. TheQIP system 2005 may correspond to a quantum computer implementation ofthe computer device 1810 in FIG. 18 and/or a quantum computer thatreceives and implements the optimized results produced by running theoptimization application 1830 (e.g., the proposed optimizer oroptimization algorithms) in the computer device 1810 in FIG. 18.

The QIP system 2005 represents a trapped atomic ion version of a quantumcomputer and can include a source 2060 that provides atomic species to achamber 2050 having an ion trap 2070 that traps the atomic species(e.g., trapped ions) once ionized by an optical controller 2020. Opticalsources 230 in the optical controller 2020 may include one or more lasersources that can be used for ionization of the atomic species, control(e.g., phase control) of the atomic ions, for fluorescence of the atomicions that can be monitored and tracked by image processing algorithmsoperating in an imaging system 2040 in the optical controller 2020.

The imaging system 2040 can include a high resolution imager (e.g., CCDcamera) for monitoring the atomic ions while they are being provided tothe ion trap 2070 (e.g., for counting) or after they have been providedto the ion trap 2070 (e.g., for monitoring the atomic ions states). Inan aspect, the imaging system 2040 can be implemented separate from theoptical controller 2020, however, the use of fluorescence to detect,identify, and label atomic ions using image processing algorithms mayneed to be coordinated with the optical controller 2020.

The QIP system 2005 may also include an algorithms component 2010 thatmay operate with other parts of the QIP system 2005 (not shown) toperform quantum algorithms (e.g., QFT, quantum simulations) that makeuse of the optimization techniques described above. The algorithmscomponent 2010 may provide instructions to various components of the QIPsystem 2005 (e.g., to the optical controller 2020) to enable theimplementation of quantum circuits, or their equivalents. That is, thealgorithms component 2010 may allow for mapping of different computingprimitives into physical representations using, for example, the trappedions in the ion trap 2070 as qubits.

The QIP system 2005 and/or the algorithms component 2010 can also beused to implement quantum simulations associated with chemical ormolecular applications. For example, in general, the Hamiltonian of aquantum object, such as a molecule, may be represented as:

=Σ_(p,q) h _(pq) a _(p) ^(†) a _(q)+Σ_(p,q,r,s) h _(pqrs) a _(p) ^(†) a_(q) ^(†) a _(r) a _(s),

where p, q, r, and s represent integers and/or labels for electronorbitals, a_(p) ^(†) and a_(q) ^(†) are the creation operators, anda_(q), a_(r), and a_(s) are the annihilation operators. The Hamiltonianmay be transformed into a form that is more amenable to quantumcomputing via, for example, Jordan-Wigner Transformation: a_(p)^(†)=Î^(⊕p−1)⊕{circumflex over (σ)}+⊕{circumflex over (σ)}_(z) ^(⊕N−p)and a_(q)=Î^(⊕q−1)⊕{circumflex over (σ)}−⊕{circumflex over (σ)}_(z)^(⊕N−q).

For small t (e.g., significantly smaller than 1), the transformation maybe approximated by the Trotter splitting process, whose first orderformula is

$e^{\frac{{- i}\;\Sigma_{j}^{n}{\hat{\mathcal{H}}}_{j}t}{\hslash}} \approx {e^{\frac{{- i}{\hat{\mathcal{H}}}_{1}t}{\hslash}}e^{\frac{{- i}{\hat{\mathcal{H}}}_{2}t}{\hslash}}e^{\frac{{- i}{\hat{\mathcal{H}}}_{3}t}{\hslash}}\ldots\;{e^{\frac{{- i}{\hat{\mathcal{H}}}_{n}t}{\hslash}}.}}$

If t is large, the expression becomes

$e^{\frac{{- i}\;\Sigma_{j}^{n}{\hat{\mathcal{H}}}_{j}t}{\hslash}} = {e^{\frac{{- i}\;\Sigma_{j = 1}^{n}{{\hat{\mathcal{H}}}_{j}{(\frac{t}{r})}}r}{\hslash}} = \left( e^{\frac{{- i}\;\Sigma_{j = 1}^{n}{{\hat{\mathcal{H}}}_{j}{(\frac{t}{r})}}}{\hslash}} \right)^{r}}$

such that

$e^{\frac{{- i}\;\Sigma_{j = 1}^{n}{{\hat{\mathcal{H}}}_{j}{(\frac{t}{r})}}}{\hslash}}$inside the parentheses may be Trotterized with

$\frac{t}{r}{\operatorname{<<}1}$for sufficiently large r. Each individual Hamiltonian

may be in the transformed form, such as the Jordan-Wigner transformedform (see above).

In some implementations, implementing the Hamiltonian of a molecule mayinclude the use of different types of gates, such as two-qubit quantumgates. A two-qubit quantum gate, such as the two-qubit CNOT gate, may bemore resource intensive than a one-qubit quantum gate, such as theHadamard gate at the physical level of implementation. Therefore,reducing the number of two-qubit quantum gates may be important forpre-fault tolerant quantum computing.

In FIG. 21, an example of

, an effective Hamiltonian operator, that frequently arise in quantumchemistry simulations is shown. The quantum circuit that implementse^(−iΣθ) where Σ is the sum of product of Pauli matrices whose addends2110 appear in a sequence 2100 of FIG. 21, using Trotter formulas hasone or more two-qubit quantum gates. While in this example, odd numbersof σ_(x) and/or σ_(y) are considered, even numbers of such matrices in aproduct, e.g., that may arise from considering Hamiltonian dynamicssimulations, may also be considered in what follows. Reordering theaddends 2110 may allow Σ to be expressed with the addends 2110 appearingin the order shown in a sequence 2150 of FIG. 21. The first quantumcircuit that implements e^(−iΣθ) with addends ordering shown in thesequence 2100 and the second quantum circuit that implements e^(−iΣθ)with addends ordering shown in the sequence 2150 may perform identicalfunctions up to the Trotter error, but the second quantum circuit withaddends shown in the sequence 2150 may utilize fewer two-qubit quantumgates by using certain reduction subroutines, examples of which aredescribed in more detail below.

Subroutine 6. Controlled Gate Cancellation (2)

In some implementations, the addend 2110 h in the sequence 2150 mayinduce a quantum circuit 2200 shown in FIG. 22A.

In some implementations, the addend 2110 g in the sequence 2150 mayinduce a quantum circuit 2230 of FIG. 22A. FIGS. 22A-B illustrate thecancellation routine of two-qubit quantum gates according to an aspectof the present disclosure. The cancellation routine may preserve thefunctionalities. The first quantum circuit 2200 may include 6 CNOT gatesand a single-qubit z-rotation gate, and the second quantum circuit 2230may include 6 CNOT gates and a single-qubit z-rotation gate. Wheneverthe originating term includes {circumflex over (σ)}_(x) on a certainqubit, the 6 CNOT and 1 R_(Z) circuit may be conjugated by H on thequbit line. Whenever the originating term includes {circumflex over(σ)}_(Y) on a certain qubit, the 6 CNOT and 1 R_(Z) circuit may beconjugated by P^(†)H from the left and HP from the right on the qubitline. The first quantum circuit 2200 may be executed before the secondquantum circuit 2230.

Still referring to FIG. 22A, in non-limiting examples, two adjacent Hgates 2260 may be cancelled using the H gate cancellation operation.Similarly, two adjacent H gates 2262 may be cancelled using the H gatecancellation operation. Next, the CNOT gate 2264 may be commuted(temporarily) with a combination 2266 of a first H gate, a P gate, and asecond H gate using the HPH commutation operation. The HPH commutationmay switch the order of execution between the combination 2266 of theHPH gates and the CNOT gate 2264. After the commutation of the CNOT gate2264 and the combination 2266 of HPH gates, a control qubit of the CNOTgate 2264 may be adjacent to a control qubit a CNOT gate 2268. A targetqubit of the CNOT gate 2264 may be adjacent to a target qubit of theCNOT gate 2268. The CNOT gates 2264, 2268 may both be cancelled usingthe CNOT gate cancellation operation.

Turning to FIG. 22B, in certain examples, two-qubit quantum gates in thefirst quantum circuit 2200 and the second quantum circuit 2230 may becancelled. The combination 2266 of HPH gates may be commuted with a CNOTgate 2270 using the HPH commutation operation. After the commutation ofthe CNOT gate 2270 and the combination 2266 of HPH gates, a controlqubit of the CNOT gate 2270 may be adjacent to a control qubit a CNOTgate 2272. A target qubit of the CNOT gate 2270 may be adjacent to atarget qubit of the CNOT gate 2272. The CNOT gates 2270, 2272 may bothbe cancelled using the CNOT gate cancellation operation.

Still referring to FIG. 22B, in some implementations, a combination of afirst H gate, a P^(†) gate, and a second H gate (i.e., HP^(†)H) may betransformed into a combination of a first P gate 2274, a H gate, and asecond P gate 2276 (i.e., PHP) using a HP^(†)H transformation(HP^(†)H→PHP). The first phase gate 2274 may be commuted with a CNOTgate 2278 and the second P gate 2276 may be commuted with a CNOT gate2280 separately using the P gate commutation operation. After the two Pgate commutation operations, a combination 2282 of the CNOT gate 2278, aH gate, and the CNOT gate 2280 may be replaced by a combination 2284including three H gates, a P^(†) gate, a CNOT gate, and two P gatesusing the CNOT gate substitution. In certain examples, a first H gate, aP gate, and a second H gate (i.e., HPH) may be transformed into a firstP^(†) gate, a H gate, and a second P^(†) gate (P^(†)HP^(†)) in a HPHtransformation (HPH→P^(†)HP^(†)). In some examples, an P^(†) gate may becommuted with a CNOT gate (not shown) using the P^(†) gate commutation.

In the present non-limiting example shown in FIGS. 22A-B, 6 CNOT gates(i.e., two-qubit quantum gates) have been reduced to 1 CNOT gate (6:1reduction ratio). Other reduction ratios may be possible, such as 5:1,4:1, 3:1, or 2:1 for example, if different ordering is used or certainoperations may not be substituted due to hardware constraints. Thecontrolled gate cancellation subroutine may maintain the functionalitiesof the quantum circuits while reducing the number of two-qubit quantumgates (e.g., CNOT gates). Reduction in the number of two-qubit quantumgates may conserve resources used in implementing the quantum circuits.

In some implementations, it may be difficult or inefficient to directlyimplement a controlled gate, such as the CNOT gate, in hardware. Asshown in FIG. 23, for example, one or more Mølmer-Sørensen (MS) gatesand/or one or more single rotation gates may be used to implement theCNOT gate based on an expression 2300, where s=±1 is the sign of theinteraction parameter specified by the ions of gate applies to, and v=±1may be chosen arbitrarily. Similarly, a MS gate may be implemented byfour H gates, two CNOT gates, and a single-qubit z-rotation gate basedon an expression 2350.

In some examples, even when the number of two-qubit gates may remain thesame or increase, if the second, equivalent circuit allows for morefrequent application of small angle MS gates while reducing the numberof CNOT gates (or MS gates with large angle, such as

compared to the first, original circuit, it may be desirable toimplement the second circuit. For example, the second circuit mayexhibit better performance in the presence of realistic noise sourcesincluding one or more of thermal noise, imprecise control, etc, in thecase where small angle MS gates are less error prone to such errorsources than large angle MS gates.

Other examples of implementing the operations described above may alsoreduce the number of two-qubit quantum gates. The subroutine 6 and theoperations described above may be implemented, for example, by thecomputer device 1810 and/or the QIP system 2005.

FIG. 24 is a flow chart that illustrates a process or method 2400 forreducing two-qubit quantum gates in quantum circuits in accordance withaspects of this disclosure. Aspects of the method 2400 may be performedby hardware and/or software in the computer device 1810 shown in FIG. 18and/or the QIP system 2005 in FIG. 20.

At block 2405, the method 2400 includes receiving a netlist includinginformation relating to a first plurality of two-qubit quantum gatesthat form the quantum circuits. The information may include informationrelating to one or more of controlled gates, R_(z) gates, Hadamardgates, phase gates, or inverse phase gates. For example, the computerdevice 1810 may receive the netlist with include information relating tothe quantum circuits configuration.

At block 2410, the method 2400 includes performing a controlled gatecancellation operation on the information relating to a first pluralityof two-qubit quantum gates to produce a second plurality of two-qubitquantum gates that is functionally equivalent to the first plurality oftwo-qubit quantum gates, wherein a first number of two-qubit quantumgates in the first plurality of two-qubit quantum gates is larger than asecond number of two-qubit quantum gates in the second plurality oftwo-qubit quantum gates. Examples of the controlled gate cancellationoperation may include the operations associated with FIGS. 22A-B. Theoptimization application 1830 may execute the operations associated withFIGS. 22A-B to reduce the number of two-qubit quantum gates.

At block 2415, the method 2400 includes generating a new netlistcontaining information about the second plurality of two-qubit quantumgates.

At block 2420, the method 2400 includes providing the new netlist toimplement a functionality of the quantum circuits based on the secondplurality of two-qubit quantum gates.

In alternative implementations, the method 2400 includes performing oneor more H gate cancellation operations.

In alternative implementations, the method 2400 includes performing oneor more HPH commutation operations.

In alternative implementations, the method 2400 includes performing oneor more CNOT gates cancellation operations.

In alternative implementations, the method 2400 includes performing oneor more HP^(†)H transformations.

In alternative implementations, the method 2400 includes performing oneor more P gate commutation operations.

In alternative implementations, the method 2400 includes performing oneor more HPH transformations.

In alternative implementations, the method 2400 includes performing oneor more P^(†) gate commutation operations.

In alternative implementations, the method 2400 includes performing oneor more CNOT gate substitution.

Although the present disclosure has been provided in accordance with theimplementations shown, one of ordinary skill in the art will readilyrecognize that there could be variations to the embodiments and thosevariations would be within the scope of the present disclosure.Accordingly, many modifications may be made by one of ordinary skill inthe art without departing from the scope of the appended claims.

What is claimed is:
 1. A method for reducing two-qubit gates in quantumcircuits, comprising: receiving information about a quantum circuit forperforming a simulation of a physical application, the informationincluding a representation of the quantum circuit by a first sequence ofaddends; reordering the addends in the first sequence of addends toproduce a second sequence of addends for representation of the quantumcircuit, the second sequence of addends conveying information relatingto a first plurality of two-qubit quantum gates that form the quantumcircuit; performing a controlled gate cancellation operation on theinformation relating to a first plurality of two-qubit quantum gates toproduce a second plurality of two-qubit quantum gates that isfunctionally equivalent to the first plurality of two-qubit quantumgates, wherein a first number of two-qubit quantum gates in the firstplurality of two-qubit quantum gates is larger than a second number oftwo-qubit quantum gates in the second plurality of two-qubit quantumgates; generating a netlist containing information about the secondplurality of two-qubit quantum gates; and providing the netlist to aquantum information processing (QIP) system to implement a functionalityof the quantum circuit to perform the simulation of the physicalapplication.
 2. The method of claim 1, wherein the information about thequantum circuit includes information relating to one or more ofcontrolled gates, R_(Z) gates, H gates, P gates, or P^(†)gates.
 3. Themethod of claim 1, wherein performing the controlled gate cancellationoperation further includes performing one or more H gate cancellationoperations.
 4. The method of claim 1, wherein performing the controlledgate cancellation operation further includes performing one or more HPHcommutation operations.
 5. The method of claim 4, wherein performing thecontrolled gate cancellation operation further includes performing oneor more CNOT gates cancellation operations.
 6. The method of claim 1,wherein performing the controlled gate cancellation operation furtherincludes performing one or more HP†H transformations.
 7. The method ofclaim 6, wherein performing the controlled gate cancellation operationfurther includes performing one or more P gate commutation operations.8. The method of claim 1, wherein performing the controlled gatecancellation operation further includes performing one or more HPHtransformations.
 9. The method of claim 8, wherein performing thecontrolled gate cancellation operation further includes performing oneor more P^(†)gate commutation operations.
 10. The method of claim 1,wherein performing the controlled gate cancellation operation furtherincludes performing one or more CNOT gate substitutions.
 11. The methodof claim 1, wherein the controlled gate cancellation operation maximizesa number of small angle Molmer-Sorensen (MS) gates in the secondplurality of two-qubit quantum gates.
 12. The method of claim 1, whereinthe physical application is a chemical application or a molecularapplication.
 13. The method of claim 1, wherein the simulation is adynamic simulation.
 14. The method of claim 1, wherein for each addendthere is a corresponding product of Pauli matrices produced by atransformation.
 15. The method of claim 14, wherein the transformationis a Jordan-Wigner transformation.
 16. The method of claim 14, whereinthe transformation is approximated by a Trotterization process.
 17. Anon-transitory computer readable medium having instructions that, whenexecuted by one or more processors, cause the one or more processors to:receive information about a quantum circuit for performing a simulationof a physical application, the information including a representation ofthe quantum circuit by a first sequence of addends: reorder the addendsin the first sequence of addends to produce a second sequence of addendsfor representation of the quantum circuit, the second sequence ofaddends conveying information relating to a first plurality of two-qubitquantum gates that form the quantum circuit; perform a controlled gatecancellation operation on the information relating to a first pluralityof two-qubit quantum gates to produce a second plurality of two-qubitquantum gates that is functionally equivalent to the first plurality oftwo-qubit quantum gates, wherein a first number of two-qubit quantumgates in the first plurality of two-qubit quantum gates is larger than asecond number of two-qubit quantum gates in the second plurality oftwo-qubit quantum gates; generate a netlist containing information aboutthe second plurality of two-qubit quantum gates; and provide the netlistto a quantum information processing (QIP) system to implement afunctionality of the quantum circuit to perform the simulation of thephysical application.
 18. The non-transitory computer readable medium ofclaim 17, wherein the information about the quantum circuit includesinformation relating to one or more of controlled R_(z) gates, H gates,P gates, or P^(†)gates.
 19. The non-transitory computer readable mediumof claim 17, wherein performing the controlled gate cancellationoperation further includes performing one or more H gate cancellationoperations.
 20. The non-transitory computer readable medium of claim 17,wherein performing the controlled gate cancellation operation furtherincludes performing one or more HPH commutation operations.
 21. Thenon-transitory computer readable medium of claim 20, wherein performingthe controlled gate cancellation operation further includes performingone or more CNOT gates cancellation operations.
 22. The non-transitorycomputer readable medium of claim 17, wherein performing the controlledgate cancellation operation further includes performing one or more HP†Htransformations.
 23. The non-transitory computer readable medium ofclaim 22, wherein performing the controlled gate cancellation operationfurther includes performing one or more P gate commutation operations.24. The non-transitory computer readable medium of claim 17, whereinperforming the controlled gate cancellation operation further includesperforming one or more HPH transformations.
 25. The non-transitorycomputer readable medium of claim 24, wherein performing the controlledgate cancellation operation further includes performing one or moreinverse P gate commutation operations.
 26. The non-transitory computerreadable medium of claim 17, wherein performing the controlled gatecancellation operation further includes performing one or more CNOT gatesubstitutions.